Originally Posted by
HillRider
Wrap a strip of paper around the bars at the widest diameter near the stem and mark the over lap with a pen. Then measure the distance between the marks. A 25.4 mm bar will have the marks 79.8 mm apart and a 31.8 mm bar will have the marks 99.9 mm apart.
+1, this is straightforward, easy to do without special tools, and plenty accurate for the job. The other option is to put a ruler next the stem. You should be able to easily eyeball the difference between 1" and 1-1/4".